1. Technical Field
The present application relates to a technique of assessing whether a speech sound has been comfortably heard or not. More specifically, the present disclosure relates to a sound pressure assessment system and the like for assessing levels of annoyance with respect to a pure tone, for the “fitting” of a hearing aid or the like to provide a sound of appropriate loudness for each individual user by adjusting the amount of amplification of sounds with respect to each frequency.
2. Description of the Related Art
In recent years, people suffering from presbycusis are increasing in number due to the aging society. Even among the young, due to increased opportunities for listening to loud music for long hours as well as other influences, there is an increasing number of people suffering from hypacusia associated with acoustic traumas. Moreover, due to the downsizing and improved performance of hearing aids, users feel less of a psychological barrier against wearing hearing aids. Against this background, there is an increasing number of users who wear hearing aids on a daily basis in order to improve their conversational aural distinction abilities.
A hearing aid is a device for compensating for the deteriorated hearing of a user by increasing the amplitude of signals of specific frequencies, among various frequencies that compose sounds that are difficult for the user to hear. The amount of sound amplification which a user desires in a hearing aid varies depending on the level of deterioration in the hearing of the user. Therefore, before beginning use of a hearing aid, “fitting” is required for adjusting the amount of sound amplification in accordance with the hearing of each user.
Fitting is performed in such a manner that the output sound pressure (i.e. fluctuations in air pressure that are perceivable as a sound) of each frequency from a hearing aid is at an MCL (most comfortable level: a sound pressure level that is felt comfortable to a user). Thus, it is considered that appropriate fitting is yet to be attained under (1) an insufficient amount of amplification, or (2) an excessive amount of amplification. For example, under an insufficient amount of amplification, aural distinction of audios is not achieved, thus falling short of the purpose of wearing a hearing aid. Under an excessive amount of amplification, although distinction of audios may be possible, there is a problem in that the user may feel annoyance, which prevents them from using the hearing aid over a long time. Therefore, a fitting needs to be done in such a manner that neither (1) nor (2) occurs. In particular, (2) has the danger of hurting the user's ears due to the possibility of presenting an excessively high sound volume from the hearing aid.
In a first step of fitting, an audiogram is measured. An “audiogram” is an evaluation of a hearing threshold value defining a smallest sound pressure of a pure tone that allows it to be heard. For example, an “audiogram” is a diagram in which, for each of a number of sounds of different frequencies, the smallest sound pressure level (decibel value) that the user can aurally comprehend is plotted against frequency (e.g., 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz).
In a next step of fitting, based on a fitting theory, which is a mathematical function for estimating an amount of amplification for each frequency, an amount of amplification for each frequency is determined. There are several kinds of fitting theories, for example: the half-gain method, in which an insertion gain of each frequency is made half of the hearing threshold value of that frequency; Berger's method, which, in addition to the above, slightly augments the amplification from 1000 Hz to 4000 Hz by taking into consideration the frequency band and level of conversational voices; the POGO method, which, based on the half-gain method, reduces the gains at 250 Hz and 500 Hz (where there is not so much speech sound information but a lot of noise component is included) by 10 dB and 5 dB, respectively; and the NAL-R method, which performs amplification so that a frequency of long-term sound analysis of words will fall around a comfortable level. There are also fitting theories which utilize not only the hearing threshold value, but also the information of an MCL and an “uncomfortable level” (hereinafter “UCL”), i.e., a sound pressure level so loud that the user feels uncomfortable, to determine an amount of gain adjustment. In that case, before determining an amount of gain adjustment, the UCL and MCL need to be measured or estimated.
The UCL is to be measured for each frequency, similarly to an audiogram. Conventionally, the UCL is measured based on subjective reporting. Specifically, a continuous sound may be presented with an ascending method (i.e., the sound pressure level is gradually increased) by using an audiometer, for example, and a sound pressure level which is so annoying (i.e., loud) that it's unbearable may be reported, this sound pressure level being measured as the UCL (Takashi KIMITSUKI et al., “Inner ear auditory testing in patients with normal hearing showing hyperacusis”, 2009).
Moreover, methods for measuring a UCL by using an electroencephalogram are under development. For example, Thornton, A. R. et al., “The objective estimation of loudness discomfort level using auditory brainstem evoked responses”, 1987 discloses a technique of estimating a UCL from the relationship between a V-wave latency in a brain stem response called the ABR (auditory brainstem response) and the intensity of a stimulation. A sound pressure level which is obtained by adding a constant (e.g., 15 or 10) to a sound pressure level at which a decrease in the V-wave latency caused by an increasing sound pressure level becomes saturated is defined as the UCL.
On the other hand, generally speaking, an MCL (most comfortable level) is difficult to be measured through subjective reporting, and therefore is often approximated as a gradient which is a half of the hearing threshold value (half gain) or a median between the UCL and the hearing threshold value.